Coupling devices of the type described above are generally known; examples include those devices described in the German Registered Design G 82 29 446.1 and G 84 04 525.6, and in EP 0 606 852 and EP 1 022 192. However, a disadvantage of these earlier devices is that if the vehicle on which they are used is subjected to severe deceleration or, in the worst case, a collision, the coupling device may slide a significant distance along the roof rail and, in the worst case, may cause it to strike the connection point between the roof rail and the roof of the vehicle. Such an event usually exerts a severe force on the load attached to the load carrier bar causing it damage. In other cases, the load carrier bar may separate from the roof rail, causing damage to the load carrier and the roof rail, as well as to the load and, in the worst case, to other things impacted by the load carrier. In certain situations, the base part may also be tilted forward in the direction of travel of the vehicle causing the rear edge of the contact surface to lift which can result in severe point contact between the front edge of the contact surface and the roof rail potentially causing damage to the roof rail or causing the base part to twist about the rail which can cause even greater damage.
Therefore there exists a need for a coupling device of the type described in the introduction, and which does not slide a significant distance along the roof rail or twist about the rail in the event of severe deceleration.